Patients with constipation-predominant irritable bowel syndrome (IBS) are often advised to increase dietary fiber intake or use fiber supplements, also referred to as bulking agents. The rationale behind the use of fiber is based on its ability to decrease gastrointestinal (GI) transit time (for treatment of constipation) and to decrease intracolonic pressure (to lessen pain). In an evaluation of the literature, 4 of 13 trials reported im- provement of constipation with bulking agents, but no effect on abdominal pain. The addition of bran has been shown to exacerbate symptoms of abdominal pain and bloating in up to 55% of patients with IBS. When treated with a psyllium preparation, 39% of patients reported improvement, while 22% of patients reported aggravation of symptoms. If fiber is derived from a food source or supplement for a patient with constipation-predominant irritable bowel syndrome (IBS), it is gradually added to the daily regimen up to a total of 25 to 30 grams per day. Fiber powder is mixed with 8 to 10 ounces of water for immediate intake, followed with an additional 8 to 10 ounces of water. Patients should be advised that it may take up to three days for fiber to provide a therapeutic effect. Fiber supplements should not be taken within three hours of certain medications, including digoxin, warfarin, or salicylates, because of the potential for binding and decreased absorption of the co-administered drug.
Stimulant laxatives (e.g., senna, bisacodyl) are sometimes used for constipation-predominant IBS, but chronic usage can lead to laxative dependence. Theoretically, stool softeners may provide benefit by increasing the water content of stool, but there is a lack of clinical data to support their use.